1. Field
One embodiment of the present invention relates to a recordable optical recording medium for recording and playing back information by optical changes in the transmittance, reflectance, etc. of a recording layer induced by the application of a light beam.
2. Description of the Related Art
In an optical recording medium such as a CD-R or a DVD-R using a dye material, part of a recording wavelength is optically absorbed in an organic dye thin film used in a recording layer, and the decomposition of the organic dye thin film and the physical deformation of a recording film are caused by heat generation in the recording layer due to the optical absorption, thereby achieving signal recording. Development has hitherto been made to increase recording density by reducing the wavelength of recording laser. A certain degree of density enhancement is possible when recent blue laser of about 400 nm is applied, but it has naturally started to face a limit in recording capacity. There is used a technique of, for example, multiplying the recording layer in order to achieve higher recording density, and, for example, a dual-layer DVD-R has been commercialized. However, for the multiplication of layers, the optical transmittance and optical absorptance of the recording layers have to be strictly controlled and the structure of the disc has to be optimized so that a stable recording/playback signal can be obtained. However, the techniques for producing multiple layers out of the dye material have not yet reached a practically adequate level due to technical difficulties. Therefore, the selling prices of such discs are higher than those of single-layer discs. Behind such circumstances, there has been reported a disc in which the order of forming the recording films in a layer located on a far side when viewed from a light entering surface is reversed to a conventional order in order to improve recording characteristics of the multilayer disc.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2005-339761 suggests the introduction of a metal oxide layer between a dye layer and a reflecting layer to improve recording/playback characteristics, a dye recording film needs to be formed on a reflecting film in the configuration of a recording layer, and a new protective layer needs to be created to prevent the interference between a bonding resin used in an intermediate layer and the dye material. A process of manufacturing the conventional single-layer disc using the dye material is not easily adapted to the multilayer disc, and production costs can be increased if high mass productivity and production yield are to be maintained because strict management of production is demanded.
Furthermore, when the recording layers have the multilayer structure, the quality of a recording signal easily changes due to a slight difference of optical characteristics of the materials of the respective layers, mutual optical interference, etc., so that there is a possibility that the margin of the designing of the configurations of the recording layers is decreased in the case where crosstalk between the recording layers is controlled. The technique of the multiplication of layers is fundamental to achieve the high density recording, but no clear designing guidance is provided in the present situation, and no report has been found regarding a method of controlling the interlayer crosstalk due to the multiplication of layers.